Water purification apparatus and units for use in laboratories and healthcare facilities are well known. Generally, they involve the reduction and/or removal of contaminants and impurities to very low levels. They contain a variety of technologies that remove particles, colloids, bacteria, ionic species and organic substances.
Some uses or applications for water require the water to be not just purified, but ‘ultrapurified’, i.e. to have a particularly low content of ionic impurities, typically with individual ion concentrations at less than 1 ug/l (1 part per billion or 1 ppb), and often at less than 10 ng/l (10 parts per trillion or 10 ppt). Such applications include the use of the water in the preparation of samples and standards for ultra-trace analytical techniques such as inductively-coupled plasma mass spectrometry (ICP-MS). SEMI standard F63-0309 specifies a maximum contamination of metals of 0.02-0.1 ppb (20-100 ppt) for each element for ultrapure water used in semi-conductor processing.
To achieve ionic purity approaching that of ‘absolute’ pure water, water purification equipment and systems include one or more dedicated final ion exchangers (IX), usually but not limited to containing one or more resins, typically a mixture of anion and cation resins. These resins contain a large number of active sites on which the “impurity ions” in the purified ‘feed’ water are taken up. In so doing, they replace hydrogen and hydroxyl ions which recombine to form water. These ion exchangers, (typically in the form of one or two cartridge packs), are usually included in a ‘water polisher’—a point-of-use system having a circuit fed with purified water.
If designed correctly, the water polisher functions as a very effective scavenger of residual impurity ions. However, as the capacity (i.e. number of free active sites) of the resin becomes used up (exhausted), those species that have built-up on the resin start to be released into the now ‘ultrapure’ product water. Because the impurity ions have been concentrated on the resin or resins (hereinafter “resin”) they can be eluted into the final ultrapure product water at far higher concentrations than in the water prior to polishing.
It is, therefore, of critical importance to change the pack containing the ion-exchange resin before significant release of ions into the product water occurs.
The established method for detecting the exhaustion of the ion-exchange resin is by monitoring the electrical resistivity of the purified water. Very pure water that is totally free of contaminant ions is often quoted as having an electrical resistivity (referred to as “resistivity”) of ‘18.15’ MΩ·cm at 25° C. (due to the slight dissociation of water molecules into hydrogen and hydroxyl ions). This figure is often rounded up to a more general figure of ‘18.2’ MΩ·cm at 25° C.
All resistivity measurements mentioned hereinafter are defined at the ‘standard’ temperature of 25° C. used for such measurements, and which is not therefore listed each time.
Sufficient quantities of undesired ions present in the water reduce its resistivity, and when a pre-determined drop in resistivity is detected, the cartridge pack is replaced with one containing unused resin.
This way of detecting the need to change the pack has the advantage of simplicity and low cost, but is not adequately sensitive to prevent the release of some ions, and in particular ions which are weakly held to the resin, at levels which could interfere with the very high purity applications described above. For example, FIGS. 2a and 2b of the accompanying drawings show an in-line ion exchange cartridge 2 having a resin bed 5 therein and a water stream 4 passing thereinto to provide a post purified water stream 6. When the ion exchange cartridge 2 is new, with only a portion of resin 5 used (shown as portion 8 in FIG. 2a), then FIG. 2a lists a resistivity measurement of ‘18.2’ MΩ·cm, with less than a 0.05 ppb level of sodium and less than a 1 ppb level of silica.
When a significant portion of the resin 5 in the cartridge 2 is used, shown as portion 10 in FIG. 2b, then only a small change in resistivity (to 18.1 MΩ·cm) has nevertheless now revealed significant increases in the presence of ionic species in the post water stream 6a, now being measured at 1 ppb for sodium, and 50 ppb for silica.
FIG. 3 shows a graph of one example of the change in resistivity in a purified water stream as ion exchange resin is used up in an ion exchange pack, and the increased presence of silica and total organic content (TOC)) in the product water.
Furthermore, direct resistivity measurements have the following limitations:
1/ Commercially available on-line resistivity sensors or meters (generally termed ‘line cells’) are at best only accurate to +/−0.75% i.e. +/−0.14 MΩ·cm at 18.15 MΩ·cm. The change in resistivity for a typical salt such as sodium nitrate is about 0.5 MΩ·cm for 1 ppb at 18 MΩ·cm. Therefore, even the best resistivity monitoring can only detect +/−0.28 ppb. Lower concentrations currently go undetected.
2/ Line cell measurements are also very sensitive to temperature. Whilst line cells usually have in-built correction for temperature, changes in water temperature are usually slower to show than changes in electrical resistivity (due to thermal lag), which lead to inaccuracies of readings. Electrical interference can also occur in measuring systems, such as during valve switching. Such occurrences may not affect less sensitive resistivity analyses, or analyses allowed to have more time, but they do affect ultrapure water measurements, and so mask changes in the ultrapurification that may not be detected until too late.
3/ Some species, such as those containing silicon and boron, are only slightly ionized and therefore produce much smaller contributions to the resistivity, but nevertheless contaminate the ultrapure water.
4/ The residual resistivity of pure water, nominally at ‘18.15’ MΩ·cm, is due to the very slight dissociation of water molecules into hydrogen and hydroxyl ions. However, the presence of sub-ppb levels of metal ions as shown in Table 1, and as shown graphically in the accompanying FIG. 4, actually decreases the concentration of free hydrogen ions resulting in a rise in resistivity, effectively masking the presence of the impurity ions. Thus, and as shown in Table 1, a resistivity measurement of 18.04 MΩ·cm, which is conventionally regarded as still ‘satisfactory’ for the ion exchange resin, masks higher levels of certain metal ions in the believed ‘ultrapure’ water, which is not desired.
TABLE 1Possible Concentration (ppb) at aResistivity (MΩ.cm) of:Metal18.1518.0417.8017.4016.40Sodium0.91.21.51.92.8Magnesium0.40.60.81.01.4Potassium0.41.11.72.43.8Calcium0.50.81.11.52.2Chromium0.30.60.81.21.8Iron1.11.31.72.23.2Nickel1.11.41.82.33.4Lead1.43.44.66.610.3
Users of water ultrapurification units need to know the water quality and it is usually displayed in ‘real time’ as the user is operating the unit. When the quality is of a lower resistivity than a pre-determined level, an alarm is often raised. However, as there can be many possible variations in the resistivity readings, as discussed above, the alarm point is usually set significantly below 18.2 MΩ·cm, typically at 10, 15, 16 or 17 MΩ·cm. As discussed above, an alarm point set at 15 MΩ·cm can already have significant levels of contaminants therein at the ppb and ppt levels, being unacceptable to very high quality users.
Moreover, any short term transient drops in the resistivity of ultrapure water are often smoothed over time, (and often occurring at a time when the user is not actually present with the unit to notice) resulting in a seemingly long-term acceptable resistivity level, whilst in the short term being unacceptable.
These factors make the simple ‘real-time’ monitoring of product water resistivity ineffective as a means of preventing the release of sub-ppb concentrations of trace impurities.
To change the pack in a timely fashion—before impurity ions are released—there are two current approaches in use:
1/ The ion-exchange capacity of the resin-containing pack is known; the water usage is logged and its effect on resin capacity calculated. The pack can then be changed well before the pack's ion-exchange capacity approaches exhaustion. This can be facilitated by feeding the pack with quite high-purity water and/or using large packs to ensure long life for the cartridge pack. However, large packs are undesirable, and this approach can waste substantial amounts of ion-exchange capacity. It is also subject to the normal risks associated with human error.
2/ In multistage monitoring, two ion exchange purification cartridge packs are arranged in series with an additional resistivity cell installed between the two cartridges. When exhaustion of the first stage purification pack is detected by an additional resistivity cell (required to be installed between the two cartridges), the pack is replaced with the second stage polishing pack—which is virtually unused (typically less than 2% when the intermediate resistivity reaches 1 MΩ·cm)—and a new pack fitted in the second, critical stage. This is a highly effective approach but is relatively complex with multiple packs and monitoring.